1. Field of the Invention
This invention relates to a power transmission device of a motor-driven power steering system and, more particularly, to an improvement of feeling of steering on a device equipped with a reduction gear.
2. Description of the Prior Art
In relation to the device of the above type, one technique is disclosed in Japanese Patent Application Laid-Open No. 57-47251, in which an electric motor for generating a driving force and a steering shaft are coupled by means of a reduction gear equipped with a plurality of gears.
In the case of connecting driving systems by the use of gears as is the above type, backlash or gap normally exists between the gears as shown in FIG. 2. To eliminate backlash, the degree of precision of individual parts must be increased remarkably and a much time is required for adjustment on positioning. Accordingly, at the time of commencing driving of a device having backlash, a gear only on the driving side starts to rotate because a gear on the driven side is separated from the gear on the driving side. As it advances a certain extent the gear on the driving side collides with the gear on the driven side, and thereafter both gears on the driving side and driven side rotate together.
In the case of a motor-driven power steering system, because of train or gears as above is interposed between the electric motor and steering shaft, and it can be assumed that a gear on the side of the steering shaft, to which a force is applied from a steering wheel, functions as the driving side gear and a gear on the side of the electric motor as the driven side gear provided that the device stands in the off state, there occurs collision between the gears as soon as a driver starts to turn the steering wheel.
In case the device stands in the on state, it can be assumed that the gear on the side of the electric motor functions as the driving side gear and the gear on the side of the steering shaft as the driven side gear; accordingly, collision occurs between the gears immediately after the electric motor starts to rotate in response to operation of the steering wheel by the driver. After all, irrespective of the device being in the on state or off state, collision of gears ocurs immediately after operation of the steering wheel.
Due to this collision, a shock with a rap, i.e. a vibration, is transmitted via the steering shaft to the steering wheel. As a result, the driver feels unpleasant at the beginning of turning of a handle. Such a vibration could not be avoided on the conventional motor-driven power steering mechanism.